When multiple elements collide within the same display field, the
following set of sequential calls to the collision routines occurs:

  * The system issues each call in a sorted order for GELs starting at
    the upper left-hand corner of the screen and proceeding to the right
    and down the screen.

  * For any colliding GEL pair, the system issues only one call, to the
    collision routine for the object that is the topmost and leftmost of
    the pair.